Field of the Invention
This invention relates to a tuned or tunable boring tool for suppressing vibrations caused in machining processes and, more particularly, to a tuned or tunable boring tool that utilizes a dynamic vibration absorber tuned for reducing vibration of a vibrating boring bar at multiple natural frequencies or vibration modes.
Description of Related Art
During a metalworking operation, there is relative motion between a workpiece and a cutting tool being urged against the workpiece. Specifically, the surface finish left on the workpiece by a previous pass of the cutting tool creates variation in chip thickness that, in turn, creates fluctuation of the cutting force magnitude. The relative motion between the workpiece and the tool is magnified by this fluctuation of the cutting force and may lead to an unstable condition known as chatter. Chatter is an example of self-excited vibration. As a result of this vibration, a poor quality surface finish and an out-of-tolerance finished workpiece may be produced.
Chatter may be especially problematic when the cutting tool is coupled to an elongated boring bar. A boring bar is essentially a cantilevered member which is anchored at one end and attached to the cutting tool at the other end. Boring bars are conventionally formed from a metal material, such as carbon steel. To reduce vibrations of the boring bars, cutting parameters such as speed and depth of cut may be reduced, decreasing the metal removal rate. However, this approach interferes with production output leading to low productivity.
Numerous attempts to eliminate boring bar vibration are known. One method for reducing vibration is using a boring bar fabricated from a stiffer material, such as carbide (e.g., tungsten carbide). However, carbide boring bars are more expensive than conventional steel bars. Furthermore, with carbide boring bars, although chatter and vibration are reduced by the inherently high stiffness of the carbide bar, vibration may still build to an unacceptable level. Additionally, carbide is fairly brittle and a minor impact upon the boring bar during use or setup may inadvertently damage the bar. A carbide boring bar extending between a steel adapter and steel tip portion is disclosed in U.S. Pat. No. 6,935,816 to Lee et al.
Another attempt to reduce vibration in boring bars is by attaching a dynamic vibration absorber mechanism to or within the boring bar. The dynamic vibration absorber may be sized during manufacturing to vibrate at a particular predetermined frequency to cancel vibration of the cantilevered bar. The dynamic vibration absorber may also include various mechanisms for tuning the bar, for particular applications.
A dynamic vibration absorber for use in a tunable boring bar, comprised of a cylindrical mass of a high-density material supported on resilient bushings, is disclosed in U.S. Pat. No. 3,774,730. When optimally tuned, the mass oscillates in response to vibration produced in the boring bar to cancel out vibration. The absorber may be tuned to accommodate the boring bar for the changes in the length of the boring bar and the weight of the cutting tool connected at the end of the bar. Such an adjustment is made by longitudinally urging pressure plates at opposing ends of the cylindrical mass, thereby compressing the rubber bushings against the mass, which alters the force of the rubber supports against the mass. Generally, the process of tuning the boring bar is easier for boring bars having higher natural frequencies where smaller tuning masses can be applied. Therefore, shorter and stiffer bars are typically easier to tune than longer, more flexible bars.
Tunable boring bars are typically formed from materials that can be machined, such as carbon steel, so that the bar can be fitted to accommodate the vibration absorption mechanism. Therefore, tunable boring bars generally are not made from stiffer materials, such as carbide, which cannot be machined through conventional means. In addition to tunable boring bars, some boring bars are designed with internal vibration absorber mechanisms that are not tunable. These anti-vibration bars will be referred to as AVB bars.
For both tunable and AVB bars, the dynamic vibration absorber is configured to cancel or minimize vibration of the bar at the first natural frequency of the bar, referred to hereinafter as the first mode. However, when vibration of the first mode of the boring bar is effectively canceled or minimized, the second natural frequency, referred to hereinafter as the second mode, may become dominant and cause chatter during cutting, even in light duty applications. The above-described tunable boring bars and AVB bars do not address vibration of the second mode or higher order modes.
Therefore, conventional tunable boring bars and AVB bars, as are known in the prior art, may not produce satisfactory performance for boring bars with narrower diameters or longer lengths. This limitation is problematic since, for certain cutting applications, narrow, long-length boring bars are particularly desirable. Therefore, there is a need for a tuned or tunable boring bar that can be optimized to cancel or minimize vibration of the boring bar at both the first mode and subsequent modes of vibration.